Lithography is a key process in the fabrication of semiconductor integrated circuits and many optical, magnetic, biological, and micromechanical devices. Lithography creates a pattern on a substrate-supported moldable surface so that in subsequent process steps, the pattern can be replicated in the substrate or in another material that is added onto the substrate.
Conventional lithography, referred to as optical lithography, involves applying a thin film of photosensitive resist onto a substrate, exposing the resist to a desired pattern of radiation and developing the exposed resist to produce a physical pattern on the substrate. Unfortunately, the resolution of patterns produced by optical lithography is limited by the wavelength of the exposing radiation. Moreover, as pattern features become smaller, increasingly expensive shorter wavelength equipment is required.
A major use of optical lithography is step-and-repeat lithography in the fabrication of integrated circuits. Step-and-repeat lithography replicates patterns much smaller than the substrate. Typically a patterned area, called a die, is replicated many times and can cover most of the area of the substrate. A typical die is about one inch square, and a typical substrate is an 8 or 12 inch diameter wafer. Step-and-repeat optical lithography exposes a first die on the substrate, moves the substrate to a new position for a new exposure of the next die and repeats the process many times.
Imprint lithography, based on a fundamentally different principle, is a promising technology to replace optical lithography. In imprint lithography a mold with a pattern of projecting and recessed features is pressed into a moldable surface on a substrate (typically a thin polymer film), and imprints into the film the features of the mold. After the mold is removed, the thin film can be further processed, as by removing the residual reduced thickness portions of the film to expose the underlying substrate. As compared to optical lithography, imprint lithography offers substantial advantages of high resolution, low cost and large area coverage.
A potential limitation on manufacturing using step-and-repeat imprint lithography is inferior imprinting quality and low throughput caused by the sensitivity of the step-and-repeat apparatus to pressure. The high resolution alignment stages used to step the substrate cannot take the optimal load needed for quality imprinting. As a consequence, step-and-repeat imprint lithography has been limited to low imprinting pressure of about 0.1 psi. As a result, the quality of step-and-repeat imprint patterns has been dramatically reduced. Conventional step-and-repeat imprint lithography leaves a thick residual layer having large thickness variations, provides low patterning fidelity and achieves only low throughput. Accordingly there is a need for improved methods and apparatus for step-and-repeat imprint lithography.